Corona forms on a round wire or cable when the voltage is raised to such a point that the voltage gradient near the wire is sufficiently high to break down the insulating properties of the air. The larger the wire the higher the voltage to cause corona. The corona is luminous and ionizes the air, giving it electrical conductiuity; thus corona has the effect of giving the conductor a larger diameter; and since a higher voltage is required for corona on a larger wire, a state of equilibrium is reached and corona is an equilibrium phenomenon not necessarily attended by spark-over. Since corona causes conductivity it is a cause of leakage and consequently of loss of power. It also increases temperature and decomposes the air into chemical constituents which are harmful to insulation. Engineers therefore have usually regarded corona as a dangerous phenomenon and one to be avoided by proper design. Transmission lines, for example, for the most part are designed so that their operating voltage is well below that at which corona would be formed on the conductors. Two suggestions have been made to mnake use of the properties of corona. The first is as a protective device for transmission lines. Since the coronais conducting and dissipates energy, it has been proposed to operate transmission lines relatively close to the coronaforming voltage. Whent abnormal rises of voltage due to lightning or other causes occur, corona begins, the air becomes conducting, and the high voltagre relieved.